This invention relates to the detection of axial cracks in the rotatable portion of a machine assembly. More particularly, it relates to a method for detecting such cracks while the rotor is in operation.
The rotor assembly employed in many rotating machinery applications is frequently subjected to relatively severe mechanical and thermal stresses due to any number of fairly routine operating conditions. While the rotors employed in these applications are generally designed to withstand such stresses, cracks can still develop in the rotor under certain circumstances. In most applications, a cracked rotor must be replaced or repaired to protect the equipment from further damage and to ensure continued efficient operation of the machinery. Furthermore, in some applications, a cracked rotor poses a safety hazard to personnel operating equipment. For such high speed rotating machinery as aircraft engines, for example, a cracked rotor may lead to a catastrophic failure.
However, the development and growth of a crack in the rotor portion of such rotating machinery is not at all predictable. In some extreme cases, undetected cracks in the rotor have become large enough to cause the rotor to burst in a brittle fracture mode. For such critical applications as aircraft engines, the rotor must be inspected relatively frequently in order to prevent a catastrophic failure. Inspecting the rotor-disc components of an aircraft engine using the techniques currently employed requires disassembling and reassembling the engine each time the rotor is inspected. Such a procedure is obviously costly and time-consuming. Furthermore, while these periodic inspections provide an assessment of the condition of the rotor assembly at the time the inspection is performed, there is always the risk that a crack might initiate and grow between inspections.
A number of non-destructive techniques are known in the art to detect the presence and growth of a rotor crack. These techniques include surface inspection methods, such as magnetic particle testing, eddy current testing, and dye penetrant techniques, and also include volumetric methods, such as ultrasonic testing. However, none of these techniques can be used to inspect the rotor while it is in operation. Vibration signature analysis may also be used to detect rotor cracks, and has offered some relief from the need to bring the rotor to a complete stop. However, until recently, this technique was useful only when the rotor was decelerated to nearly zero rotational velocity.
U.S. Pat. No. 4,380,172, issued Apr. 19, 1983 to I. Imam et al., and assigned to instant assignee discloses and claims an on-line vibration signal analysis method of crack detection which eliminates the need to decelerate the rotating machinery. In the method disclosed by that patent, the rotor is tested while operating at normal speed and load conditions, by transitorily perturbing the rotor so that any crack present manifests itself by producing a new and different vibration response mode. U.S. Pat. No. 4,408,294, issued Oct. 4, 1983, to I. Imam , and also assigned to the instant assignee, also discloses and claims a method for detecting rotor cracks by utilizing vibration signature analysis. In the method disclosed by that patent, vibration signature analysis is performed on a set of difference signals obtained using histogram techniques, while the rotor is operating under normal conditions.
The methods disclosed by both of the above-referenced patents employ a change in the bending stiffness of the rotor, as the rotor is rotated, to detect the presence of a crack. For "transverse" cracks, that is, cracks which are contained in a plane which is generally perpendicular to the central axis of the rotor, the development and growth of the crack produces a measurable change in the bending stiffness of the rotor. This stiffness change can be detected by the vibration signature analysis techniques disclosed by the above-referenced patents, and can be employed to indicate the presence and size of a crack in the rotor. However, for some types of rotating machinery, such as aircraft engines and the like, most of the rotor cracks are "axial" cracks. As defined herein, an "axial" crack is a crack which is contained in a plane which is generally parallel to the central axis of the rotor. Thus, the term "axial crack" includes, for example, radial-axial cracks, that is, cracks which extend in a direction generally parallel to the central axis of the rotor and which have a depth in a direction which is generally perpendicular to that axis, and also includes circumferential-axial cracks, that is, cracks which extend generally in a circumferential direction which respect to the central axis of the rotor and which have a depth in a direction which is generally parallel to that axis. For these axial cracks, changes in the bending stiffness of the rotor as it is rotated are less pronounced. The present invention provides a method and apparatus for detecting these axial cracks by utilizing the relationship between the additional unbalance force created by such cracks and the rotational speed of the rotor.
Accordingly, it is an object of the present invention to provide a method and apparatus for detecting axial cracks in the rotor assembly of rotating machinery, while the machinery is in operation.
It is another object of the present invention to provide for on-line continuous monitoring of the machinery, for early detecting of cracks in the rotor.
It is a further object of the present invention to provide detection of axial rotor cracks utilizing vibration signature analysis techniques.